Sparse channel estimation for orthogonal frequency division multiplexed signals

ABSTRACT

A computationally efficient channel estimation technique for use within an orthogonal frequency division multiplexing (OFDM) communication system determines coefficients of a channel transfer function by calculating the dot products of a pilot vector and a plurality of interpolation vectors. One dot product is preferably calculated for each subcarrier of interest within the system. The pilot vector is extracted from an OFDM symbol received from a communication channel. In a preferred approach, a number of interpolation vectors are precalculated and stored within a communication device for subsequent use during channel estimation and equalization operations. The technique is highly flexible and can be implemented using, for example, a variable user block size or a variable pilot vector size.

FIELD OF THE INVENTION

[0001] The invention relates generally to communication systems and,more particularly, to techniques and structures for performing channelestimation in such systems.

BACKGROUND OF THE INVENTION

[0002] After a communication signal has traveled through a communicationchannel, equalization is often performed on the received signal toremove channel effects from the signal. One of the channel effects thatoften needs to be removed is intersymbol interference (ISI). In awireless communication system, ISI is typically present in the form ofmultipath interference. That is, a transmit signal travels through thewireless channel via multiple different paths that each have a differentchannel delay. For example, one signal component may travel in a directpath from the transmitter to the receiver while one or more other signalcomponents are reflected from objects in the surrounding environmenttoward the receiver. As can be appreciated, the signal component thattravels directly to the receiver will typically be the first to arriveat the receiver and have the largest amplitude. The reflected componentswill typically arrive at the receiver sometime later and have smalleramplitudes. Although smaller in amplitude, the reflected signals caninterfere with the direct signal making it more difficult to accuratelydetect the data therein. Equalization is thus used in the receiver toreduce or eliminate the negative channel effects from the receivedsignal to improve the likelihood of accurate detection.

[0003] In most equalization techniques, an estimate of the presentchannel response is first determined. The channel estimate is then usedto process the received signal to remove the negative channel effects.The channel estimation process is often a computationally complex andtime consuming process. That is, performance of such processes willoften consume a large percentage of system resources and may introduceundesirable delays in the receiver processing. As can be appreciated, itis generally desirable to reduce computational complexity and processingdelays within a communication system. This is especially true withinhandheld and portable communication units that have limited processingcapabilities and a limited supply of power (e.g., batteries). Therefore,there is a need for channel estimation techniques and structures thatare computationally efficient while still providing accurate estimates.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004]FIG. 1 is a block diagram illustrating a conventional orthogonalfrequency division multiplexing (OFDM) transmitter;

[0005]FIG. 2 is a signal diagram illustrating an OFDM symbol stream thatmay be transmitted from the transmitter of FIG. 1;

[0006]FIG. 3 is a diagram illustrating a conventional OFDM receiver;

[0007]FIG. 4 is a diagram that is representative of the frequencyspectrum of a typical OFDM symbol;

[0008]FIG. 5 is a block diagram illustrating an OFDM equalizationsubsystem in accordance with one embodiment of the present invention;

[0009]FIG. 6 is a diagram illustrating the selection of pilot symbolsfrom an OFDM symbol based on a plurality of subcarriers of interest inaccordance with one embodiment of the present invention; and

[0010]FIG. 7 is a flowchart illustrating a method for performing channelestimation and equalization in an OFDM communication system inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION

[0011] In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein in connection with one embodiment may beimplemented within other embodiments without departing from the spiritand scope of the invention. In addition, it is to be understood that thelocation or arrangement of individual elements within each disclosedembodiment may be modified without departing from the spirit and scopeof the invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to which the claims are entitled. Inthe drawings, like numerals refer to the same or similar functionalitythroughout the several views.

[0012] The present invention relates to computationally efficienttechniques and structures for providing channel estimation within acommunication system implementing orthogonal frequency divisionmultiplexing (OFDM). The techniques and structures are most useful whenonly a subset of the subcarriers within each OFDM symbol are ofinterest. In a system using subcarrier division multiplexing, forexample, where subsets of the data subcarriers are assigned to users ona dynamic basis, a communication device associated with a particularuser will only be interested in the subcarriers assigned to that user.In one approach, interpolation vectors are first obtained for each ofthe subcarriers of interest. A dot product is then calculated betweeneach of the interpolation vectors and a pilot vector extracted from areceived OFDM symbol. Each dot product results in an equalizationcoefficient for a corresponding subcarrier of interest. The equalizationcoefficients are then used to modify the subcarriers of interest withinthe received OFDM symbol to reduce or remove undesirable channel effects(e.g., frequency selective fading) from the symbol. In one approach,only a subset of the pilot symbols within each OFDM symbol are used toform the pilot vector. The inventive principles are particularly wellsuited for software implementation (e.g., soft-PHY architectures),although hardware and hybrid software/hardware realizations can also beprovided.

[0013] Orthogonal frequency division multiplexing (OFDM) is amulti-carrier transmission technique that uses a plurality of orthogonalsubcarriers to transmit information within an available spectrum.Because the subcarriers are orthogonal to one another, they can bespaced much more closely together within the available spectrum than,for example, the individual channels in a conventional frequencydivision multiplexing (FDM) system. That is, the orthogonality of thesubcarriers prevents inter-subcarrier interference within the system. Ina typical OFDM system, orthogonality is achieved by using subcarriersthat each have a spectrum with a null at the center frequency of each ofthe other subcarriers. Before transmission, each of the subcarriers ismodulated with a low rate data stream. Thus, the transmitted symbol rateof the OFDM system is low and the transmitted OFDM signal is highlytolerant to multipath delay spread within the channel. For this reason,many modern digital communication systems are turning to OFDM as amodulation scheme for signals that need to survive in environmentshaving multipath reflections and/or strong interference. Many wirelesscommunication standards have already adopted OFDM including, forexample, IEEE Standard 802.11 a, the digital video broadcasting Tstandard (DVB-T), and the high performance radio local area networkstandard (HiperLAN). In addition, several industry consortia, includingthe Broadband Wireless Internet Forum and the OFDM Forum, are proposingOFDM for fixed wireless access systems.

[0014] Before a description of the inventive principles is undertaken, adiscussion of the basic operating characteristics of a conventional OFDMcommunication system is presented. It should be appreciated, however,that the inventive principles can be implemented in any communicationsystem utilizing OFDM techniques and are not limited to use withinsystems or devices having the specific architectures described below.

[0015]FIG. 1 is a block diagram illustrating a conventional OFDMtransmitter 10. In a typical scenario, the transmitter 10 will be partof a transceiver unit that is capable of supporting duplex communicationwithin a wireless communication system. As illustrated, the transmitter10 includes: a modulator 12, an inverse fast Fourier transform (IFFT)unit 14, a parallel to serial converter 16, a cyclic extension unit 18,a radio frequency (RF) transmit unit 20, and an antenna 22. Themodulator 12 receives a plurality of symbols (S₀, S₁, S₂, . . .,S_(N-1)) that need to be transmitted by the transmitter 10. Themodulator 12 uses each of the input symbols to modulate a correspondingsubcarrier of the OFDM system to generate a symbol modulated subcarrier(e.g., S_(0SC), S_(1SC), S_(2SC), . . . , S_((N-1)SC)) at an outputthereof As described above, each of the subcarriers of the OFDM systemis orthogonal to each of the other subcarriers to keep inter-subcarrierinterference to a minimum. The modulator 12 can use any of a variety ofmodulation types to modulate the subcarriers (e.g., binary phase shiftkeying (BPSK), quadrature phase shift keying (QPSK), quadratureamplitude modulation (QAM), differentially coded star QAM (DSQAM), andothers). In addition, the modulator 12 can use a different modulationtype for each individual symbol or for different groups of symbols ifdesired.

[0016] The input symbols (S₀, S₁, S₂, . . . , S_(N-1)) are used togenerate a single OFDM symbol to be transmitted by the transmitter 10.The symbol modulated subcarriers (S_(0SC), S_(1SC), S_(2SC)SC, . . . ,S_((N-1)SC) ) form a frequency domain representation of the OFDM symbol.The symbol modulated subcarriers are applied to the inputs of the IFFT14 to generate a time domain representation of the OFDM symbol. Asshown, the time domain representation of the OFDM symbol consists of aplurality of time domain samples (S₀, S₁, S₂, . . . , S_(N-1)). Any formof inverse discrete Fourier transform (IDFT) can be used to perform theinverse transform operation. The IFFT is preferred, however, because itis the most computationally efficient method available. As is wellknown, the number of time domain samples generated by the IFFT 14 isequal to the number of frequency components input thereto (i.e., N).

[0017] The samples output by the IFFT 14 are applied to the parallel toserial converter 16 which generates a sample stream representing theOFDM symbol. This serial OFDM symbol is transferred to the cyclicextension unit 18 which adds a cyclic extension (or guard interval) tothe OFDM symbol. The cyclic extension is added to the OFDM symbol toprevent the occurrence of inter-symbol interference in the channel thatcan be caused by the channel's memory (i.e., multipath reflections). Thecyclic extension usually consists of a plurality of samples (e.g., NGsamples) that are copied from the end of the serial OFDM symbol andplaced at the beginning of the symbol. The number of samples willtypically depend upon the memory of the channel. It is typicallydesirable to use a cyclic extension having a length that is no more than10% of the length of the OFDM symbol to maintain efficient (e.g., lowoverhead) operation.

[0018] The cyclic extension unit 18 outputs each OFDM symbol and itscorresponding cyclic extension in a continuous stream to the RF transmitunit 20. FIG. 2 is a diagram illustrating the stream output by thecyclic extension unit 18 in a typical application. The RF transmit unit20 is operative for converting the OFDM symbol stream into a radiofrequency signal for transmission into the wireless channel. To performthis function, the RF transmit unit 20 may include, for example, adigital to analog converter, a frequency conversion unit (e.g., an upconverter), a power amplifier, and/or any other equipment required togenerate an RF transmit signal. The output of the RF transmit unit 20 isdelivered to the antenna 22 which transmits a radio frequencycommunication signal 24 into the channel. It should be appreciated thatother processing functionality, such as error coding circuitry, may alsobe included within the OFDM transmitter 10.

[0019]FIG. 3 is a block diagram illustrating a conventional OFDMreceiver 28. Like the transmitter 10 of FIG. 1, the receiver 28 willtypically be part of a transceiver unit that is capable of supportingduplex communications within a wireless communication system. Asillustrated, the receiver 28 includes: an antenna 30, an RF receive unit32, a synchronization unit 34, a serial to parallel converter 36, an FFTunit 38, and a demodulation unit 40. The antenna 30 receives an RFcommunication signal 24 from the channel. The RF receive unit 32converts the received RF signal to a format required for subsequentprocessing. The RF receive unit 32 may include, for example, a low noiseamplifier, one or more frequency conversion units (e.g., a downconverter), an analog to digital converter, and/or any otherfunctionality required to achieve the desired signal format. The RFreceive unit 32 transfers the received signal to the synchronizationunit 34 which synchronizes the signal in a manner that allows theindividual OFDM symbols within the signal to be recognized and thecyclic extensions to be discarded. The OFDM symbols are delivered oneafter the other to the serial to parallel converter 36 which convertseach symbol into a parallel group of time domain samples (r₀, r₁, r₂, .. . , r_(N-1)). The samples are input into the FFT unit 38 whichgenerates a plurality of frequency domain symbol modulated subcarriers(R_(0SC), R_(1SC), R_(2SC), . . . , R_((N-1)SC)). The symbol modulatedsubcarriers are then demodulated by the demodulator 40 to produce aplurality of symbols (R₀, R₁, R₂, . ., R_(N-1)).

[0020] The impulse response of the wireless channel (i.e., the channelimpulse response or CIR) is typically assumed to be time-invariant forthe duration of one OFDM symbol. Thus, the time domain convolution ofthe transmitted time domain signal with the CIR is equivalent to themultiplication of the spectrum of the transmitted signal with thefrequency domain transfer function H(f) of the channel (which is simplythe Fourier transform of the CIR). Each of the frequency domain symbolmodulated subcarriers (R_(0SC), R_(1SC), R_(2SC), . . . , R_((N-1)SC))that are received by the receiver 28, therefore, is the product of acorresponding symbol modulated subcarrier (S_(0SC), S_(1SC), S_(2SC), .. . , S_((N-1)SC)) of the transmitter 10 and an associated coefficientof the frequency domain transfer function H(f) of the channel, plus someadditive channel noise (e.g., additive white Gaussian noise). This canbe expressed in equation form as follows:

R _(mSC) =S _(nSC) ×H _(n) +n _(n)

[0021] where R_(nSC) is the nth symbol-modulated subcarrier received inthe receiver, S_(nSC) is the nth symbol-modulated subcarrier transmittedby the transmitter, H_(n) is the frequency domain channel transferfunction coefficient corresponding to the nth subcarrier, and n_(n) is awhite Gaussian noise sample corresponding to the nth subcarrier. Ascoherent detection is assumed for the system, the received data symbolsR_(nSC) need to be de-faded in the frequency domain. Thus, an estimateof the frequency domain transfer function H(f) of the channel needs tobe made.

[0022] In a typical approach, the channel transfer function is estimatedusing pilot symbols that are included within the OFDM symbolstransmitted by the transmitter 10. The pilot symbols are usually locatedat fixed frequency intervals within the OFDM symbols. FIG. 4 is adiagram illustrating the frequency spectrum of an OFDM symbol 42 havinga plurality of subcarriers. As shown, every fourth subcarrier within theOFDM symbol 42 includes a pilot symbol 44 that can be used for channelestimation. The other subcarriers within the OFDM symbol 42 are used tocarry user data symbols through the channel. The spacing of the pilotsymbols 44 within a particular OFDM symbol will typically depend uponthe specific system being implemented. Similarly, the overall number ofsubcarriers within each OFDM symbol will also be system specific. Thelocation and content of the pilot symbols within the transmitted OFDMsymbol will typically be known within the receiver.

[0023] When an OFDM symbol is received by the receiver 28, the pilotsymbols are extracted from the received signal. The extracted pilotsymbols include information about the frequency domain transfer functionof the channel (e.g., coefficients) at the frequencies of the pilotcarrying subcarriers. To obtain information about the frequency domaintransfer function of the channel at the frequencies of the data carryingsubcarriers, interpolation techniques are often used. In the simplestapproach, linear interpolation is performed from pilot to pilot. Thismethod provides reasonable performance as long as the inverse of therealized delay spread does not approach the pilot spacing. Often,however, the linear approach falls short of the performance levelsrequired in modem communication systems.

[0024] In an “optimal” interpolation method, zero-padded FFTs (or DFTs)are used to fill in the missing transfer function coefficients. Thismethod will typically result in the most accurate channel estimateavailable, but it is very computationally complex. For example, in atypical procedure, the pilots from a received OFDM symbol are input toan FFT to generate an array of values at an output of the FFT. Thenumber of values within the array output by the FFT is equal to thenumber of pilots input to the FFT (i.e., M). A plurality of zeros isthen added to the array of values to increase the total number of valuesto the number of subcarriers within the OFDM symbol (i.e., from M to N).This process is known as zero-padding. An inverse N-point FFT is thenperformed on the zero-padded array to achieve interpolated channeltransfer function coefficients for all of the subcarriers. Thesecoefficients can then be used to perform channel equalization for thedata carrying subcarriers within the received OFDM symbol on asubcarrier by subcarrier basis (e.g., by simple division).

[0025] In conceiving the present invention, it was appreciated that manysituations exist where only a subset of the subcarriers within aparticular OFDM symbol are of interest. For example, in an OFDM systemwhere orthogonal frequency division multiple access (OFDMA) is beingutilized, individual users are each assigned subsets of the subcarrierson a dynamic basis. Thus, the communication equipment associated with aparticular user (e.g., a handheld communicator) will only be concernedwith the corresponding subcarriers assigned to that user and not theentire subcarrier array. In a similar example, a basestation in an OFDMsystem implementing OFDMA will typically receive OFDM signals frommultiple user terminals concurrently. The base station must thenestimate the channel associated with each user terminal separately. Eachchannel estimation, therefore, is only concerned with the subcarriers ofinterest for the corresponding user terminal. Other situations alsoexist where only a subset of the subcarrier array is of interest. Inthese situations, it will often be inefficient to perform thecomputationally complex optimal interpolation technique described aboveto determine equalization coefficients for the subcarriers of interest.Therefore, in accordance with the present invention, techniques arepresented that are capable of providing a significant reduction in thecomputational complexity associated with channel estimation (i.e., withrespect to the optimal interpolation approach) when only a subset ofsubcarriers within an OFDM symbol are of interest. The techniques of thepresent invention will typically provide a performance level betweenthat of the linear interpolation method and the optimal interpolationmethod (often closer to the optimal method).

[0026] The reduced complexity channel estimation techniques of thepresent invention are derived from the optimal interpolation approachdescribed above. In the optimal approach, a pilot vector having a lengthM (extracted from a received OFDM symbol) is transformed into a vectorof length N using zero padded FFTs (or DFTs). An M-point FFT is firstperformed on the pilot vector x, as follows:$X_{f} = {\sum\limits_{n = 0}^{M\quad 1}\quad {x_{n} \cdot ^{{- {j2\pi}}\quad {{fn}/M}}}}$

[0027] where X_(f) are the frequency domain coefficients output by theFFT, M is the number of pilots in the pilot vector, n is the time index,and f is the frequency index. The resulting group of coefficients X_(f)is then zero-padded to length N (i.e., total number of subcarrierswithin the OFDM symbol) with the zeros inserted in the high frequency(center) terms. An N-point inverse FFT (or DFT) is then performed on thezero-padded array to generate a plurality of interpolated equalizationcoefficients x_(n)′ that can be described as follows:${\hat{x}}_{n} = {{\sum\limits_{f = 0}^{{({M/2})} - 1}\quad {X_{f} \cdot ^{{j2\pi}\quad {{fn}/N}}}} + {\sum\limits_{f = {N - {({M/2})}}}^{N - 1}\quad {X_{f - N + M} \cdot ^{{j2\pi}\quad {{fn}/N}}}}}$

[0028] The subscript of X has been modified in the second summation ofthis equation to conform to the original, non-zero padded indiciesdescribed above. Substituting the second equation into the first andexpanding the result yields:${\hat{x}}_{k} = {{^{j \cdot 0} \cdot {\sum\limits_{n}\quad {x_{n} \cdot ^{{- j} \cdot 0}}}} + {^{{j2\pi}\quad {k/N}} \cdot {\sum\limits_{n}\quad {x_{n} \cdot ^{{- {j2\pi}}\quad {n/M}}}}} + {^{{j2\pi}\quad {{k2}/N}} \cdot {\sum\limits_{n}\quad {x_{n} \cdot ^{{- {j2\pi}}\quad {{n2}/M}}}}} + \cdots + {^{{{j2\pi}{({{({M/2})} - 1})}}{k/N}} \cdot {\sum\limits_{n}\quad {x_{n} \cdot ^{{- {j2\pi}}\quad {{n{({{({M/2})} - 1})}}/M}}}}} + {^{{j2\pi}{({N - {({M/2})}})}} \cdot {\sum\limits_{n}\quad {x_{n} \cdot ^{{- {j2\pi}}\quad {{n{({N - {({M/2})} - N + M})}}/M}}}}} + \cdots + {^{{{j2\pi}{({N - 1})}}{k/N}} \cdot {\sum\limits_{n}\quad {x_{n} \cdot ^{{- {j2\pi}}\quad {{n{({M - 1})}}/M}}}}}}$

[0029] where the index k has been introduced to distinguish the indiciesof x′ from the indicies of x. It was determined by rearranging the termsof this equation (to combine x_(n)) that each element of x′ is simplythe dot product of x (the pilot vector) with an interpolation vector Ihaving length M. For arbitrary k and N, the interpolation vector I canbe calculated as follows:$I_{n} = {{\sum\limits_{i = 0}^{M - 1}\quad ^{{{j2\pi}\quad {i{({{({k/N})} - {n/M}})}}})}} + {\sum\limits_{i = {N - {({M/2})}}}^{N - 1}\quad ^{{j2\pi}{({{({i \cdot {k/N}})} - {({{({i - N - M})} \cdot {n/M}})}})}}}}$

[0030] In accordance with one aspect of the present invention, aplurality of interpolation vectors are predetermined (using the aboveequation or some variant thereof) and stored within a communicationdevice. When an OFDM symbol is received by the communication device, apilot vector is extracted from the OFDM symbol. Interpolation vectorsare then retrieved for each subcarrier of interest within thecommunication device and a dot product is calculated between the pilotvector and each of the retrieved interpolation vectors. Each dot productresults in an equalization coefficient for a corresponding subcarrier ofinterest. This basic approach will be referred to herein as the vectorinterpolation method (VIM). The equalization coefficients resulting fromthe dot products are then used to equalize the associated subcarriersignals within the OFDM symbol (e.g., by division). As long as thesubcarriers of interest remain unchanged, the same interpolation vectorscan be used to process each OFDM symbol received from the channel.

[0031] The VIM offers a computational flexibility that was notpreviously available using the optimal interpolation approach. Forexample, the VIM allows flexibility in block size (i.e., the number ofsubcarriers computed) and pilot vector size that was not previouslyavailable. Using the VIM, an OFDM communication system can beimplemented that allows the number of subcarriers assigned to each userto be dynamically varied during system operation. Similarly, an OFDMsystem can be provided that allows the total number of subcarriersand/or pilot symbols within each OFDM symbol to be dynamically varied.Theoretically, the optimal interpolation method can be performed withonly a subset of the pilot symbols within an OFDM symbol to reducecomputational complexity. However, to dynamically vary the size of thepilot vector used to perform optimal interpolation during systemoperation, block processing elements (e.g., FFTs) would have to beavailable for each expected vector size. As dedicated hardware istypically used to perform the FFTs using the optimal approach, thiscould easily become cost prohibitive. For this reason, systemsimplementing the optimal approach typically employ the entire array ofpilots to perform the interpolation. For a given number of pilots used,the optimal method and the VIM will provide identical performance.However, when the number of subcarriers to be interpolated is a subsetof those traversed by the pilot tones used in the channel estimation, asignificant computation benefit is achieved by using the VIM. Thus, theVIM allows channel estimation to be performed only in the region of thesubcarriers of interest in a relatively simple and dynamic fashion,usually with a reduced computational requirement. In addition, the VIMis particularly well suited for implementation within a soft-PHYarchitecture.

[0032]FIG. 5 is a block diagram illustrating an OFDM equalizationsubsystem 50 in accordance with one embodiment of the present invention.The subsystem 50 will typically be implemented as part of a wirelessreceiver (e.g., receiver 28 of FIG. 3) in an OFDM communication system.In a multi-user basestation scenario, a separate equalization subsystem50 can be provided for each user currently communicating with thebasestation. It should be appreciated that the individual blocks withinthe block diagram do not necessarily correspond to discrete hardwarestructures. For example, one or more of the blocks (or all of theblocks) may be implemented in software within a digital processingdevice.

[0033] As illustrated, the equalization subsystem 50 includes: asubcarrier tracking unit 52, a pilot vector unit 54, an interpolationvector retrieval unit 56, a computation unit 58, an equalizer 60, and amemory 62. The subcarrier tracking unit 52 tracks the currentsubcarriers of interest for the subsystem 50. The pilot vector unit 54extracts a number of pilot symbols from a recently received OFDM symbol64 to form a pilot vector. The interpolation vector retrieval unit 56retrieves a plurality of interpolation vectors corresponding to thesubcarriers of interest identified by the subcarrier tracking unit 52from the memory 62. As will be described in greater detail, theinterpolation vector retrieval unit 56 may use information about thepilot symbols within the pilot vector to determine which interpolationvectors to retrieve. The computation unit 58 uses the pilot vector fromthe pilot vector unit 54 and the interpolation vectors from theinterpolation vector retrieval unit 56 to generate a plurality ofequalization coefficients for delivery to the equalizer 60. Theequalizer 60 then uses the equalization coefficients generated by thecomputation unit 58 to equalize each of the subcarriers of interestwithin the corresponding OFDM symbol 64. This procedure is repeated foreach OFDM symbol received.

[0034] As described above, the subcarrier tracking unit 52 is operativefor tracking the subcarriers of interest within the equalizationsubsystem 50. To do this, the subcarrier tracking unit 52 will typicallyneed to determine which subcarriers are presently assigned to a userassociated with the subsystem 50. In a basestation that is servicingmultiple users within the OFDM system, a separate equalization subsystem50 may be provided for each currently connected user. The subcarriertracking unit 52 within each subsystem 50 would thus track thesubcarriers assigned to the corresponding user. The subcarrier trackingunit 52 will output an indication of the present subcarriers of interestto the interpolation vector retrieval unit 56 and possibly the pilotvector unit 54. In at least one approach, the number and location of thesubcarriers of interest associated with a particular user can changewith time. The subcarriers of interest associated with a particular userare not necessarily adjacent to one another within the OFDM symbol.

[0035] In one embodiment of the present invention, the pilot vectorgenerated by the pilot vector unit 54 includes all of the pilot symbolsfrom the present OFDM symbol 64. In a preferred approach, however, thepilot vector unit 54 includes selection functionality for dynamicallyselecting a subset of pilot symbols within the OFDM symbol 64 to be usedto form the pilot vector (i.e., to perform the interpolation). By usinga subset of pilot symbols, computational complexity can be reducedconsiderably. However, as discussed previously, a reduction inequalization performance will usually be experienced. In this regard, atradeoff can be made between performance and computational efficiency.The subset of pilot symbols that is used within the vector interpolationcalculations should envelope all of the subcarriers of interest beingprocessed. For example, with reference to FIG. 6, if subcarriers 66, 68,70, 72, and 74 are presently of interest, then at least pilot symbols A,B, C, and D should be used in the interpolation. The pilot vector unit54 can determine which of the pilot symbols to use for a particular setof subcarriers of interest based on the interpolation vectors that areknown to be available within the memory 62.

[0036] The memory 62 will include a plurality of interpolation vectorsfor use during the channel estimation process. The interpolation vectorscan all have the same length or a plurality of different lengthinterpolation vectors can be provided. The interpolation vectors willtypically be calculated a priori using the interpolation vector equationdescribed above or a similar equation. In a system where the number andarrangement of the subcarriers of interest can vary, the length of thecorresponding pilot vectors may also vary. In this scenario,interpolation vectors can be stored in the memory 62 for each subcarrierwithin each possible pilot vector arrangement. The interpolation vectorretrieval unit 56 retrieves the appropriate interpolation vectors fromthe memory 62 based on the subcarriers identified by the subcarriertracking unit 52. If the pilot vector unit 54 dynamically selects pilotsymbols for the pilot vector, the interpolation vector retrieval unit 56will also use information about the present pilot vector to retrieve theappropriate interpolation vectors. As described previously, theretrieved interpolation vectors will each have the same length as thepilot vector assembled by the pilot vector unit 54. In a preferredapproach, the computation unit 58 calculates a separate dot productbetween the pilot vector and each of the retrieved interpolationvectors. The result of each dot product is the equalization coefficientfor the corresponding subcarrier of interest. The equalizer 60 equalizesthe subcarriers of interest within the present OFDM symbol 64 bydividing each subcarrier by the corresponding equalization coefficient.The equalized data symbols are then output for further processing.

[0037]FIG. 7 is a flowchart illustrating a method for performing channelestimation and equalization in an OFDM communication system inaccordance with one embodiment of the present invention. A plurality ofsubcarriers of interest are first identified (block 70). A pilot vectoris then extracted from a received OFDM symbol (block 72). The pilotvector can include all of the pilot symbols from the OFDM symbol or asubset thereof. In at least one embodiment, pilot symbols are chosen forthe pilot vector based on the quantity and location of the subcarriersof interest. An interpolation vector is obtained for each of theidentified subcarriers of interest (block 74). In a preferred approach,the interpolation vectors are retrieved from a memory within thecorresponding communication unit. However, other methods for obtainingthe interpolation vectors can also be used. A dot product is nextcalculated between the pilot vector and each of the interpolationvectors (block 76). Each dot product results in an equalizationcoefficient for a corresponding subcarrier of interest. Eachequalization coefficient is then applied to a corresponding subcarrierwithin the received OFDM symbol (block 78). This process is repeated foreach OFDM symbol received.

[0038] It should be appreciated that the principles of the presentinvention can be beneficially implemented in any receiver unit usedwithin an OFDM-based communication system. The receiver unit can belocated, for example, within a multi-user basestation, a single userhandheld communicator, a satellite uplink, downlink, or crosslinktransceiver, a transceiver supporting a terrestrial wireless link,mobile transceivers within ad hoc networks, and in a wide variety ofother communication device types. The inventive principles haveapplication in both wireless and wired systems, although wirelesssystems will typically derive the greatest benefit.

[0039] Although the present invention has been described in conjunctionwith certain embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art readily understand.Such modifications and variations are considered to be within thepurview and scope of the invention and the appended claims.

What is claimed is:
 1. A method for performing channel estimation withina communication system implementing orthogonal frequency divisionmultiplexing (OFDM), comprising: receiving an OFDM symbol from acommunication channel, said OFDM symbol having a plurality of datasubcarriers and a plurality of pilot symbols; identifying subcarriers ofinterest; generating a pilot vector using pilot symbols from said OFDMsymbol; obtaining a first interpolation vector corresponding to a firstsubcarrier of interest; and calculating a dot product of said pilotvector and said first interpolation vector to generate an equalizationcoefficient for said first subcarrier of interest.
 2. The method ofclaim 1, comprising: obtaining an interpolation vector corresponding toeach subcarrier of interest; and calculating a dot product of said pilotvector and an interpolation vector for each subcarrier of interest togenerate an equalization coefficient for each subcarrier of interest. 3.The method of claim 1, wherein: generating a pilot vector includesselecting a set of pilot symbols from said OFDM symbol based upon theidentities of said subcarriers of interest.
 4. The method of claim 1,wherein: generating a pilot vector includes using all pilot symbolswithin said OFDM symbol.
 5. The method of claim 1, wherein: obtaining afirst interpolation vector includes selectively retrieving said firstinterpolation vector from a memory.
 6. The method of claim 1, wherein:identifying subcarriers of interest includes identifying subcarriersassociated with a first user within the communication system.
 7. Acommunication device for use in a communication system implementingorthogonal frequency division multiplexing (OFDM), comprising: means forreceiving an OFDM symbol from a communication channel, said OFDM symbolhaving a plurality of subcarriers and a plurality of pilot symbols;means for extracting a group of pilot symbols from said OFDM symbol toform a pilot vector; means for acquiring an interpolation vectorassociated with a first subcarrier of interest; and means for performinga mathematical operation using said interpolation vector and said pilotvector to generate a first equalization coefficient for said firstsubcarrier of interest.
 8. The communication device of claim 7, wherein:said means for performing a mathematical operation includes means forcalculating a dot product of said pilot vector and said interpolationvector.
 9. The communication device of claim 7, comprising: means foracquiring an interpolation vector associated with each of a set ofsubcarriers of interest; and means for calculating a dot product of saidpilot vector and each of said interpolation vectors acquired by saidmeans for acquiring to generate equalization coefficients for said setof subcarriers of interest.
 10. The communication device of claim 9,wherein: said subcarriers within said set of subcarriers of interest areassociated with a single user within the communication system.
 11. Thecommunication device of claim 7, wherein: said communication device is aportable communicator.
 12. The communication device of claim 7, wherein:said communication device is a communication base station.
 13. Thecommunication device of claim 7, wherein: said communication deviceincludes a wireless OFDM transceiver.
 14. The communication device ofclaim 7, wherein: said means for acquiring an interpolation vectorincludes means for selectively retrieving an interpolation vector from amemory.
 15. The communication device of claim 7, wherein: said means forextracting a group of pilot symbols includes means for extracting all ofsaid pilot symbols in said OFDM symbol for inclusion within said pilotvector.
 16. The communication device of claim 7, wherein: said means forextracting a group of pilot symbols includes means for extracting asubset of said pilot symbols in said OFDM symbol for inclusion withinsaid pilot vector.
 17. The communication device of claim 7, wherein:said means for acquiring an interpolation vector and said means forperforming a mathematical operation are each implemented in softwarewithin a digital processing device.
 18. The communication device ofclaim 7, comprising: means for processing a first subcarrier of interestwithin said OFDM symbol using said first equalization coefficient.
 19. Acommunication device for use in a communication system implementingorthogonal frequency division multiplexing (OFDM), comprising: areceiver to receive an OFDM symbol from a communication channel, saidOFDM symbol having a plurality of subcarriers and a plurality of pilotsymbols; a subcarrier tracking unit to track subcarriers of interest; apilot vector unit to assemble a pilot vector using pilot symbols fromthe OFDM symbol; an interpolation vector retrieval unit to retrieve aninterpolation vector for each of said subcarriers of interest from amemory; and a computation unit to determine a channel estimate usingsaid pilot vector and said interpolation vectors retrieved by saidinterpolation vector retrieval unit.
 20. The communication device ofclaim 19, wherein: said subcarrier tracking unit tracks subcarriersassociated with a particular user.
 21. The communication device of claim19, wherein: said pilot vector unit selects pilot symbols from the OFDMsymbol based on said subcarriers of interest indicated by saidsubcarrier tracking unit.
 22. The communication device of claim 19,wherein: said pilot vector unit assembles pilot vectors of varyinglength.
 23. The communication device of claim 19, wherein: saidinterpolation vector retrieval unit retrieves interpolation vectors thateach have a length that is equal to that of said pilot vector.
 24. Thecommunication device of claim 19, wherein: said computation unitincludes a digital processor to calculate a dot product of said pilotvector and an interpolation vector.
 25. A computer readable mediumhaving program instructions stored thereon for implementing a method todetermine a channel estimate within an orthogonal frequency divisionmultiplexing (OFDM) communication system when executed within a digitalprocessing device, said method comprising: determining a set ofsubcarriers of interest; forming a pilot vector using pilot symbols froman OFDM symbol; obtaining a first interpolation vector corresponding toa first subcarrier of interest; and calculating a dot product of saidfirst interpolation vector and said pilot vector to generate anequalization coefficient for said first subcarrier of interest.
 26. Thecomputer readable medium of claim 25, wherein: determining a set ofsubcarriers of interest includes identifying a user and determining aset of subcarriers assigned to said user.
 27. The computer readablemedium of claim 25, wherein: forming a pilot vector includes selectingpilot symbols for inclusion within said pilot vector based on said setof subcarriers of interest.
 28. The computer readable medium of claim25, wherein: obtaining a first interpolation vector includes retrievingsaid first interpolation vector from a memory.
 29. The computer readablemedium of claim 25, wherein said method comprises: obtaininginterpolation vectors for each subcarrier within said set of subcarriersof interest; and calculating a dot product of said pilot vector and eachof said interpolation vectors to generate equalization coefficients foreach subcarrier within said set of subcarriers of interest.